Ultrashort laser pulses have been used to modify the refractive index of transparent materials, such as glasses of various compositions, for the trimming of optical waveguides and fabrication of fiber-based optical devices, such as fiber Bragg gratings. Laser-based trimming techniques have been utilized in connection with planar waveguides formed via well-known photolithography, diffusion, and etching techniques. Such techniques are useful for modifying the refractive index profile of the pre-existing, planar waveguide. For example, the ultrashort laser pulses are typically applied to modify the optical path length of the waveguide.
Pulse energy, pulse width, scan rate, and repetition frequency have been identified as process parameters relevant to determining the nature and extent of the trimming operation. For example, the shape of a pre-existing waveguide has been modified via application of ultrashort waveguide pulses to taper a portion thereof. Other ways in which the trimming technique has been used to locally alter the physical structure of a pre-existing waveguide include adjusting the polarization behavior of the waveguide to create approximately symmetric regions of index change. Such techniques may involve writing index changes within the pre-existing waveguide that are slightly laterally displaced from each other. See U.S. Patent Application Pub. No. 20020085824 A1, published Jul. 4, 2002, and entitled “Index trimming of optical waveguide devices using ultrashort laser pulses for arbitrary control of signal amplitude, phase, and polarization.”
The above-identified, prior techniques have generally been directed to the modification of pre-existing waveguides that are already capable of guiding light of a desired wavelength. Moreover, the waveguides have been typically fabricated using non-laser-based methods, such as photolithography. These methods are fairly limited to the surface of the substrate, in turn typically limiting the functionality of devices fabricated thereby to the two-dimensional interactions in that planar surface.
Another complication involves the wavelength of the optical signal. Wavelength can be a limiting factor for any waveguide—whether at the surface or in the bulk—because certain waveguides that guide light at one wavelength may be incapable of guiding light at longer wavelengths. As the telecommunications industry migrates to systems based on optical signals having wavelengths at or near 1.55 microns, the optical waveguides and other devices that were effective in different regimes, such as 800 nm, may no longer provide suitable performance or, in some cases, may be incapable of guiding light at all.